Brazing sheets are conventionally used as material for heat exchangers of automobiles and the like, the brazing sheet being formed of an aluminum alloy (hereinafter sometimes simply referred to as the “Al alloy”) as a core material, with a brazing filler material formed on one or both sides of the core material (hereinafter sometimes simply referred to as the “brazing sheet”). Brazing sheets each including a core material and a brazing filler material made of an Al—Si based alloy cladded on at least one side of the core material are used as each member, for example, tubes, header plates, side supports and the like of heat exchangers such as an inter cooler, a radiator, an oil cooler, a capacitor, a heater core, and an evaporator used in automobiles. After forming each of these brazing sheets into a predetermined shape, each member is bonded during a brazing step.
To prevent cracking during casting, namely, refining of crystal grains, it was common to add an Al—Ti—B based alloy in aluminum alloys. However, coarse Si particles are easily formed when Ti and B are added. A solidified structure of a 4000 series aluminum alloy brazing filler material is mainly composed of an Al—Si eutectic and an a solid solution, and crystallization of coarse Si particles having a maximum diameter of more than 20 μm may occur in the eutectic structure. The Si concentration and size of coarse Si particles do not significantly vary depending on the subsequent working and heat treatment. Therefore, it is known that formation of coarse Si particles causes significant melting around the coarse Si particles during heating for brazing, leading to formation of molten holes. Meanwhile, the formation of coarse Si particles was conventionally suppressed by a refining treatment of eutectic Si which is performed by adding Na, Sr and the like (see Patent Document 1). However, this technique was insufficient for suppression of the formation of coarse Si particles.
Therefore, Patent Document 1 discloses a brazing filler material of an aluminum brazing sheet for heat exchangers, which enables prevention or suppression of the formation of molten holes during heating for brazing by preventing crystallization of coarse Si particles or suppressing the growth of coarse Si particles; and a method for manufacturing the same.